A glass substrate used in fabrication of a flat panel display such as a liquid crystal display, a plasma display or an electroluminescence display is manufactured from a glass plate larger in size than the glass substrate as a raw material (hereinafter, such a glass flat e will be referred to as “mother material”). The glass substrate is manufactured by means of a method of cutting a mother material. For example, as for a glass substrate for a liquid crystal display used in a 15 inch monitor, as described in FIG. 1, four pieces of the substrate are obtained from a mother material of 550 mm×650 mm in size. Glass substrates can be obtained by cutting one mother material. In such a case, a glass plate on a surface of which a film is formed is used as a mother material and glass substrates can be obtained by cutting it.
On the other hand, in a case where a glass substrate for use in a small liquid crystal display is manufactured, it is an important cutting method of directly cutting a glass substrate obtained by adhesion. The reason therefor is that the glass substrate for use in a liquid crystal display is of a structure in which two sub-glass substrates are adhered to each other with a sealing agent. The glass substrate for used in a liquid crystal here has cells 1 in which a liquid crystal is sealed between the two sub-glass substrates.
Cutting of a mother material is disclosed in the following non-patent literatures: Yasuaki Miyake, “Scribe and Brake Technique”, Monthly FPD Intelligence, a special number entitled LCD Fabrication and Inspection Technique in the 4th generation, published by Press Journal Inc. on Jan. 20, 2000, pp 85-89) (hereinafter referred to as “non-patent literature 1”); and Takeshi Yamamoto, “Light Beam Orientation Device Capable of Use in Clean Room and Laser Glass Cutting Device,” Monthly FPD Intelligence 1994. 4,” published by Press Journal Inc., pp 28-31) (hereinafter, referred to as “non-patent literature 2”). The cutting of a mother material disclosed in the non-patent literatures 1 and 2 includes: a scribing step of forming a scribe line on a surface of the mother material; and a step of applying a mechanical or thermal stress on the scribe line. This cutting method is called a scribe and brake method.
A cutting method described in the non-patent literature 1 is a general scribe and brake method. This scribe and brake method includes a scribe line forming step and a mechanical stress applying step. The scribe line forming step is a step of scratching a mother material surface with a diamond or sintered carbide wheel cutter to form a scribe line. A depth of a scribe line formed in this step is a value of the order in the range of from 10 to 15% of the thickness of a mother material. A mechanical stress applying step is a step of applying a mechanical stress on a site where a scribe line is formed. In this step, glass substrates are cut from the mother material.
A cutting method described in the non-patent literature 2 includes a scribe line forming step and a thermal stress applying step. This cutting method is a method adopted in a glass cutting apparatus from ACCUDYNE Co. in USA. The scribe line forming step includes the sub-steps of: (1) forming a small physical damage at an end of a mother material with a metal foil; (2) illuminating the physical damage with a linear laser beam; and (3) spraying a mixed gas of helium and water onto the mother material immediately after the illumination with the laser beam to rapidly cool it. In the scribe line forming step, a scribe line on a molecular level is formed in the mother material. The thermal stress applying step is a step of illuminating both sides of a scribe line with laser light, followed by spraying the mixture of helium and water onto the scribe line to thereby cool it. By laser beam illumination and cooling immediately thereafter, a thermal stress arises at the scribed line to cut a glass substrate from the mother material.
In the cutting method described in the non-patent literature 1, however, the following problems have remained unsolved. A first problem is that cracks occur in a mother material surface and in the vicinity of the mother material surface in formation of a scribe line. The cracks easily lead to glass cullet both in scribing and cutting. A second problem is that a number of cracks occur at an intersection between scribe lines in formation of the scribe lines. Breakage of a mother material occurs in cutting because of the presence of the cracks with ease. The glass substrates obtained by cutting are weak in edge strength thereof because of the presence of the cracks. A third problem is that, in order to improve an edge strength of a glass substrate, it is required to chamfer a glass substrate. In order to remove glass powder and the like accompanied by chamfering, a necessity arises for cleaning. That is, the cutting method is problematic because of a low productivity as a cutting method of a glass substrate.
On the other hand, the cutting method described in the non-patent literature 2 has been expected to have an effect of being unnecessary for a clean process or a chamfering process after the cutting, therefore with a high productivity. A problem has been still remained, however, in the cutting method because of a low cutting precision and a reduced speed in cutting process. Moreover, a cutting apparatus from ACCUDYNE Co. disclosed in the non-patent literature 2 is expensive, therefore, the apparatus cannot be widely proliferated in general use.
It has been understood that glass breakage occurs from a fine physical damage originally present on a mother material surface. Therefore, with reduction in surface physical damage on a surface of the glass substrate itself, it contributes to more improvement on glass strength and more prevention of glass breakage. In a case where a polishing process is applied that removes a physical damage on a surface of the glass substrate, it lowers productivity in cutting of a glass substrate. On the other hand, if reduction can be achieved in physical damage on a glass surface during the cutting process of a mother material, the productivity in cutting of a glass substrate may be improved. Therefore, there is a need for reducing a physical damage on a mother material surface during the cutting process.